This invention relates to controlling the relative movements of a tool and a workpiece in a multiple-axis machine, for example a three-axis cutting machine.
In typical controllers for such machines, information about a desired sequence of cutting tool movements is provided from a punched tape or a host computer. The actual movements along the three axes are then produced by motors or electromechanical actuators which are powered in accordance with control signals delivered by the controller. Position encoders sense the actual positions of the tool and generate position feedback signals for use by the controller.
Traditionally, the motors are of the linear torque DC type driven by analog drive amplifiers.
With digital processor-based numerical controllers, two approaches are taken to converting the digital output of the processor to an appropriate form for driving the machine.
In one approach the linear torque DC motors are replaced by stepper motors, and digital output pulses from the processor step the motor in an open loop system. The stepper motor advances a fixed amount with each pulse so that the number and rate of output pulses directly control the position and speed of the machine.
In the other approach, linear torque motors were used in a feedback loop in which the digital commands from the processor were converted first to analog drive values, and then the analog drive values were converted to pulse width modulated form. Feedback was provided by a position encoder to the digital processor, and by an independent analog tachometer to a pulse width modulation or DC power amplifier.